Greenland ice melt speed has doubled

New study fills gap in our knowledge of the origin of sea level rise

The Upernavik Glacier in northwest Greenland is melting into a lake. Greenland's ice sheet has been melting twice as fast during the 21st century as it did during the 20th. (Niels Jákup Korsgaard/Natural History Museum of Denmark)

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Greenland's ice sheet melted twice as fast between 2003 and 2010 as it did from 1900 to 1983, according to the first study of Greenland ice loss over the past century that is based on observations rather than models.

Between 1900 and 2010, meltwater from Greenland boosted the global sea level by 25 millimetres, making up 10 to 18 per cent of global sea level rise, reports the new study published today by an international team of researchers, including some in Canada.

The new study also shows exactly where ice has disappeared from. Generally, the places that were melting during the 20th century are the same ones that are still melting now. (Nicolaj Krog Larsen/Aarhus University)

The new study, published Wednesday in Nature, provides the first good estimate of how much Greenland has contributed to sea level rise so far, filling a gap in the last report by the United Nations Intergovernmental Panel on Climate Change, the researchers report.

That will help make models — and future predictions — of sea level rise more reliable, said Kristian Kjeldsen, lead author of the paper. Kjeldsen is a postdoctoral researcher at the University of Ottawa and at the Centre for GeoGenetics at Natural History Museum of Denmark at the University of Copenhagen.

To do that, they turned to 3D or stereophotogrammic aerial photos captured between 1978 and 1987. The images clearly show a mark called a trimline. Below it, the landscape is a different colour, where the glaciers scraped against the rock and eroded away the vegetation. The trimline is the maximum extent of glaciers before they started retreating around 1900, the end of a cold period called The Little Ice Age.

By using the trimline as a reference and combining the 4,000 aerial photos with later satellite data and "control points" of known elevation, the researchers were able to measure and reconstruct the extent of the ice in 1900, and how much it changed over time.

Kangiata Nunata Sermia in southwest Greenland shows different lines that marks the extent of the ice at different points in time. The upper trimline (transition) between the lighter and darker valley sides marks the extent of the glacier during the Little Ice Age, while the lower lines shows the extent at later points in time. (Hans Henrik Tholstrup/Natural History Museum of Denmark)

"What we have is an estimate that's really founded in observation," said Kjeldsen.

Previously, estimates of ice lost during the 20th century came from computer models based on recent satellite data.

The new study also shows exactly where ice has disappeared from. Generally, the places that were melting during the 20th century are the same ones that are still melting now, Kjeldsen said.

"The pattern agreed so much going back in time."

Overall, from 2003 to 2010, Greenland dumped 186 billion tonnes of meltwater a year into the world's oceans, the researchers estimated.

From 1900 to 1983, the rate averaged 75 billion tonnes a year – much lower, but still significant, Kjeldsen said.

"That was a little bit surprising, that it was so consistent and so high," he said.

Still, he said, "The mass loss has really increased quite a bit going into the 21st century."

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